1. Field of the Invention
The present invention relates to an image pickup optical system of a small size, more particularly to a compact image pickup optical system exerting a high performance and suitable for digital apparatuses (digital still camera, digital video camera and the like) for fetching a projected image of a photographic subject using a solid imaging element.
2. Description of the Related Art
In recent years, along with the diffusion of a personal computer and the like, a digital still camera, a digital video camera and the like (hereinafter, simply referred to as “digital camera”) capable of easily fetching image information into a digital apparatus have been increasingly made available to individual users. It is expected that the digital cameras will be more penetrated in the future as an input device for the image information.
As another trend, a solid imaging element such as CCD (Charge Coupled Device) installed in the digital camera has been increasingly reduced in size, in response to which it becomes necessary for the digital camera to be further downsized. Accordingly, it is strongly demanded that an image pickup optical system occupying a largest area in the digital input device be also downsized. The easiest way of downsizing the image pickup optical system is to downsize the solid imaging element. However, it is necessary to downsize a light receiving element in order to downsize the solid imaging element, which makes it more difficult to manufacture the solid imaging element and requests the image pickup optical system to exert a higher performance.
When the image pickup optical system is downsized with the solid imaging element remaining the same size, an exit-pupil position is inevitably drawn to an image surface. When the exit-pupil position is drawn to the image surface, an off-axis luminous flux emitted from the image pickup optical system obliquely enters the image surface. Therefore, a condensing performance of a micro lens provided in a front surface of the solid imaging lens cannot be sufficiently exerted, which unfavorably generates a significant difference between an image brightness at a central part of the image and an image brightness in a peripheral part of the image. An effort of increasing a distance between the exit-pupil position of image pickup optical system and the solid imaging element in order to solve the foregoing problem unavoidably leads the entire image pickup optical system to grow in size.
As additional points to be remarked, there is a growing demand for the cost reduction of the image pickup optical system due to the competitive price reductions in recent years, and a higher performance is requested in the image pickup optical system because of an increasingly higher density of the solid imaging element in recent years. In order to satisfy the aforementioned recent demands, image pickup optical systems for solid imaging element comprising three lenses were proposed in U.S. Pat. Nos. 6,795,253, 6,804,067, No. 2003/210341, No. 2004/21957, and No. 2005/30645 of the US Patents.
However, any of the image pickup optical systems comprising three lenses, which are recited in the foregoing patent literatures, comprises two positive lenses and a negative lens and accordingly included a disadvantage in downsizing the image pickup optical system.